In transmitting data over a twisted pair, there have been proposed a number of different techniques, especially those for generating line codes for transmission systems, such as two-loop HDSL. The line codes that have been utilized have included echo-canceled and FDM versions of QAM/CAP, PAM, and DMT. These are all provided in various T1E1 contributions. However, one of the benchmarks for transmission over the two-loop HDSL is referred to as the 6 dB margin CSA (carrier serving area) range. Some of the techniques are described in K. Schneider, "A Modulation Technique for CSA Range HDSL2," HDSL Study Project for T1E1.4 Technical Subcommittee Working Group Members, Feb. 3-7, 1997, which is incorporated herein by reference.
Although substantially all of the modulation methods for CSA have fallen short of the 6 dB range, they have been combined with various encoding/decoding techniques to increase their range. One such modulation method is the Overlapped PAM Transmission with Interlocking Spectra (OPTIS), this being a modulation method for CSA range HDSL2 transmission. This proposed approach has purported to achieve an uncoded SNR margin in excess of 1 dB for all provisionally agreed crosstalk environments, as well as mixed crosstalk scenarios. This is combined with a 5 dB forward error correction code, a trellis code, to provide an overall 6 dB of coded performance margin on CSA loops. However, it is very difficult to achieve the 5 dB forward error correction code, even with a trellis technique. It is relatively easy to achieve 4 dB forward error correction, but an additional 1 dB is considerably more difficult. Therefore, the 1 dB uncoded SNR margin is marginal at best when realizing the difficulty of achieving the 5 dB for error correction improvement. The OPTIS technique is described in M. Rude, M. Sorbara, H. Takatori, and G. Zimmerman, "A Proposal for HDSL2 Transmission: OPTIS" Standards Project: T1E1.4:HDSL2, Jun. 30-Jul. 2, 1997, which is incorporated herein by reference.
The OPTIS transmission technique utilizes an iteratively determined HDSL2 transmit spectrum, one for the downstream data, and one for the upstream data. It is noted that the transmit spectra is defined as a set of "templates" which are basically filters that define the frequency spectrum. By so shaping the frequency spectrum, the desired transmission technique can be achieved. However, as noted above, even the 1 dB uncoded SNR is marginal at best when considering that the benchmark is a 6 dB CSA range.
When a communication system utilizing twisted pair loops is implemented, it must be realized that a plurality of these loops with be "bundled" with each other. There can therefore exist crosstalk between systems that operate on identical transmission mode, and there can be additional problems when there are two different transmission modes that are being transmitted down twisted pairs in the same line. In any event, if the crosstalk from adjacent lines within a bundle presents a noise error to the system, this will decrease the SNR of the system. Therefore, various techniques have been implemented that will reduce the input of crosstalk.